(CNN) -- Britain on Monday downgraded its international terror threat level from "severe" to "substantial."

Britain raised its threat level to "critical" in 2007 after a bomb attack at Glasgow Airport.

The notice was issued by The Joint Terrorism Analysis Center (JTAC), which operates independently of ministers.

According to the British Home Office Web site, the change means that a terrorist threat remains a strong possibility and an attack might occur without further warning.

JTAC assesses the threat from international terrorism by considering all relevant intelligence and information.

Its decision is based on a broad range of factors, including the currently assessed intent and capabilities of international terror groups such as al-Qaeda.

Home Secretary Alan Johnson said in a statement: 'The change in the threat level to substantial does not mean the overall threat has gone away -- there remains a real and serious threat against the United Kingdom and I would ask that the public remain vigilant.

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"The decision to change the threat level...is based on the very latest intelligence, considering factors such as capability, intent and timescale. Substantial continues to indicate a high level of threat; and that an attack might well occur without further warning. The threat level is kept under constant review."

There are five levels of terror threat, from "low," indicating that an attack us unlikely, to "critical," which warns of an imminent attack.